Regarding PL refit sag, where would the weak points be?
The nacelle struts themselves aren't likely to deform, as their cross-section is hollow, with a higher moment of inertia (IMHO) than the Ertl solid struts. Also, they won't shear along the seams if glued properly. So they're solid.
Where the strut meets the nacelle?
The joint at the top of the strut doesn't experience much (axial?) torque, since it's located close to the centerline of the nacelle. So it's not likely to contribute to sag.
Will the secondary hull deform under the torque?
Maybe the secondary hull will deform inward slightly along the, er, "waterline", but then again it may not, as the shuttlebay assembly should provide support there. If not, I'll place some support between the hull and the shuttlebay. I don't think the sec hull would deform at the dorsal spine either, since the torque would pull on either side of the spine, and the top piece of the sec hull is one piece, not two like the Ertl. (If it were two pieces, we'd have a load on the center seam there.) So I don't think the secondary hull will deform. I also don't think the dorsal piece would deflect upward but I might add something inside the hull to connect the dorsal spine to the shuttlebay ceiling and/or walls for reinforcement.
How about the bottom end of the strut?
So we're left with the joint where the strut meets the secondary hull to cause deflection and sag. No surprise. Yep, there's torque here, but I think that because of the push/pull two-joint design of the internal connection, the load is actually transferred to the secondary hull pieces, sort of like a monocoque. If the sec hull pieces are glued properly, I can't see them shearing along the joint, and as I wrote, I can't see the hull deforming either.
Now an armature certainly couldn't hurt, but I wouldn't assume
that these things will sag. The design is different from the Ertl design, transferring the load in some clever ways.
Now all this is just idle speculation until an engineer gets a chance to publish an analysis of this thing. That would be interesting.